Why does the oil drop in Millikan's oil drop experiment fall so slowly?
The oil drops fall so slowly (a) because they are small and (b) because they are attracted to a positive plate above them.
Ionizing radiation gave the fine oil droplets a negative charge.
Millikan could measure the rate at which a drop fell through the view of the telescope.
He could then change the charge on the plates so that the drop would be attracted to the positive plate above it.
He could adjust the voltage to keep the drop stationary. Other drops with different masses and charges either moved upwards or continued to fall.
This gave him enough information to calculate the charge on the drop.